Relative Impacts of Snow and Sea Surface Temperature Anomalies on an Extreme Phase in the Winter Atmospheric Circulation.

In association with extreme anomalies in the extratropical atmosphere, numerical experiments using an atmospheric general circulation model are performed to investigate the relative impact of the anomalous snow with SST anomalies on the atmospheric circulation. Large negative anomalies in the Eurasian snow cover and global SST anomalies observed in 1988/89 are employed as the respective boundary forcings because winter atmospheric states largely shifted in 1989.The model is integrated for half a year from 1 September. Five-member ensemble states are obtained by conducting the light snow (LSNW) run, in which the snowfall was suppressed over eastern Eurasia during the first 3 months with prescribed SSTs, and another experiment, which employed observed SST anomalies instead of snow anomalies (the SST run). The LSNW run simulated dipole (positive in midlatitudes and negative in polar regions) anomalies in 500-hPa height similar to those observed in 1989, although the amplitude was smaller over the North Pacific. Surface warming over Eurasia found in winter 1989 is also reproduced through albedo feedback. On the other hand, the SST run reveals large height anomalies over the North Pacific in addition to the significant dipole similar to that in the LSNW run, but failed to reproduce observed surface warming as well as negative snow anomalies over Eurasia. SST anomalies in the equatorial Pacific corresponding to La Niña in 1988/89 are responsible for simulated height anomalies over the North Pacific in the SST run, whereas an influence of extratropical SST anomalies appears to be tenuous relative to either tropical SST anomalies or Eurasian snow anomalies. The amplitude of response in the LSNW run is roughly 60% of that in the SST run.An analysis of the dynamics emphasizes that, in the upper troposphere, interactions of anomalies themselves with climatological zonal asymmetries as well as changes in transient eddy activities contribute to the height response found in the model. This suggests that the nonlinearities in the atmosphere are also important in addition to the snow and SST anomalies for the extreme anomalies in winter 1989 atmospheric circulation.